1. Field of the Invention
The invention relates to projection displays, and more particularly to a technique for faster color display from liquid crystal display (LCD)-generated images.
2. Description of the Related Art
With the ever increasing graphical nature of computer user interfaces, improving image display devices by improving the size and quality of displayed images generated by digital and analog signals is important. The two most popular image display devices are the cathode ray tube (CRT) and the LCD. Although large screen CRTs are available, they often are bulky. Slimmer screens can be made using various image generating devices, such as the LCD, polymer dispersed liquid crystal displays (PDLCDs), or other LCD technology, but, at present, screen size is limited by manufacturing considerations.
Projection of an image created by digital or analog signals, rather than by direct imaging, can be an efficient and economical way to increase display size, provided the overall system size is practical and image quality is acceptable. Projection of color images, however, presents some problems associated with speed limitations of the particular device used to convert the image signal to an image that can be displayed.
In color displays, there are three major systems for producing different colors and color brightness. One system uses color-specific pixels, in which each pixel transmits only one of the red, green, or blue components needed for a full color image. In this system, the pixels are arranged in groups of red, green, and blue. A particular color is achieved in an area of the pixels by turning "on" or "off" the appropriate pixels in that area. For example, if purple is the desired color, the green pixels in the area would remain off while the red and blue pixels would be turned on. Displayed image brightness may also be controlled by turning the pixels on and off. If bright purple is desired, for example, the red and blue pixels in the area would remain on for longer periods of time than for a less bright purple. The red and blue pixels are turned on and off at a higher rate for the bright purple than for the less bright purple. The greater the percentage of "on" time, the brighter the color.
Another color display system is similar to the system described above in that each pixel transmits only one of the primary colors. In this second system, the pixels are arranged in groups of red, green, and blue. To achieve a particular color in a pixel area, appropriate pixels in the area are turned on or off. Brightness is controlled by varying the amount of light transmitted by an "on" pixel, rather than by turning off some of the pixels. This system provides better resolution than a system that leaves pixels unilluminated to achieve shades of displayed color.
As will be appreciated, systems that rely on color-specific pixels may significantly diminish image resolution compared to systems that employ any of their pixels to create an image at any one time, irrespective of color requirements. Moreover, systems that are subpixelated, color-specific, and that have a limited subpixel size may exhibit diminished image resolution compared to systems that have smaller subpixel size. At some point, however, reducing subpixel size may be prohibitive in terms of cost. Cost may also be a problem for systems that use three imagers (e.g., LCDs), one each for red, green, and blue light, and a dispersing element, such as a prism, to separate the colored light from white.
One technique that avoids both subpixelation and the use of three imagers in colored displays is known as field sequential color. Field sequential color systems comprise the other major system for producing different colors and color brightness. Each pixel transmits or reflects red, green, and blue light sequentially in time. When the sequence is transmitted sufficiently fast, the human brain integrates and perceives the three light colors as a single blended color, determined, to a certain extent, by the relative proportion of the color inputs. If the transmission is not fast enough, however, the image may appear smeared and the color integration incomplete, causing so-called "rainbow effects." To reduce such effects, the three colors must be sequenced at a relatively high rate within a video frame. For example, with a frame rate of approximately 60 to 200 Hz, the corresponding three-color (subframe) change rate must be approximately three times this range, or 180 to 600 Hz.
In sequential color displays, color hue and brightness are usually controlled in the time domain. This arises because most LCDs or other imagers capable of the necessary speeds are bi-stable devices, not analog. Digital devices provide only fully-on and fully-off periods, while analog devices can vary the intensity substantially continuously between the fully-on and fully-off states. For example, with a digital device, if a bluish-purple hue is desired from a certain pixel for a time period, the pixel is electronically controlled to transmit blue light longer than red light, and to transmit no green light during that time period. Pulse width modulation may be used to provide such electronic control. Adding pulse width modulation to the requirement for high speed sequencing may, however, present a practical limitation, because the LCD must be capable of very high on-off switching rates. As an example, assume that 24-bit color is provided, i.e., 8-bits or 256 color levels or values each for red, green, and blue. If analog LCDs could be used, then at approximately 300 to 600 Hz, a different analog voltage level would have to be applied to each pixel, each voltage assuming one of 256 values corresponding to the 256 color values. Simultaneously, a color filter would have to be switched to pass light corresponding only to the color to be displayed. The problem, as noted above, is that most analog displays are too slow for this type of system.
Some ferroelectric LCDs (or FLCDs), on the other hand, are bi-stable devices, and are capable of very high switching rates. They can be used to provide the equivalent of analog color levels via pulse width modulation switching in the time domain. For example, each 300th to 600th of a second subframe interval may be further divided into 6-bit time slots, or 64 time divisions using pulse width modulation. To display at 1/64th color intensity with a filter, such as a blue filter, the FLCD would be turned on for 1/64th of the 300th to 600th of a second subframe that the blue filter is engaged. Although 6-bits per color, or a total of 18-bits for three colors, can effectively be displayed with a single currently available FLCD, 24-bit color cannot be reliably displayed. As a result, trade-offs must be made, either in frame rate, color separation, or color capability. Additional information about the technical difficulties involved in using FLCDs in field sequential color systems may be found in a technical disclosure entitled "FLC/VLSI Display Technology," dated Dec. 1, 1995, published by Displaytech, Inc., which is incorporated by reference herein in its entirety.
It is noteworthy that Texas Instruments has developed a so-called "one-color plus two-color" field sequential color system that uses digital or deformable micromirror devices (DMDs) capable of 24-bit color as imagers. The Texas Instruments system, however, uses two DMDs for addressing lamp spectrum issues, while not addressing imaging speed or bandwidth issues.
The present invention is directed to providing a full color, image producing system that uses available FLCDs, while avoiding some of the aforementioned design trade-offs.